I bought a footlocker, but just can't figure out how the included lock works. I'll be using my own padlock, but it would be nice to have the built in one working, too. 

The cylinder appears to be plastic, and the black tab above it unlatches the lock when pushed down. I can turn the key freely over and over, but it just never locks. Is anybody familiar with this type of lock that could offer some assistance? Thanks!